Present day toners are formulated from a range of potential components. Most toner compositions include at least a polymeric binder material and a colorant. Other commonly used components include black and colored magnetic oxides, charge control agents, internal additives to augment toner properties, such as aiding in deagglomeration and homogeneous distribution of the colorant in the toner composition, and external additives, to aid in the proper function of the toner. The components used in a particular toner formulation are dependent on the requirements of the machine in which the toner is ultimately intended to be used. For instance, the toner formulation must take into account such parameters as image quality, reliability, carrier life, toner shelf life, etc., all of which are intricately involved with the mechanical capability and design of the hardware of the machine. Often, there is more than one component of a toner formulation which performs to eradicate certain undesirable properties of the toner. These same components may however, also contribute to other problems, or the combination of two or more components which affect the same toner properties may result in over-correction of a problematic area in the toner performance. Therefore, the combination of components selected to comprise a given toner composition must be carefully balanced, taking into account the full range of toner performance parameters which may be affected by each component and the interaction of each component with every other component of the toner composition, and the machine and its various components and systems.
Given that each of the foregoing parameters will affect toner performance in some manner, it is unlikely that any one toner will achieve optimum performance in all areas. Therefore, toner producers determine which parameters are most critical to the performance of a toner for a given purpose and which may be compromised, and to what extent.
Toner performance is determined by the combination of components, and by the physical, electrical and chemical properties of each. Such properties include pigment dispersion, particle size, particle size distribution, particle shape, bulk density, mechanical strength, flow properties, triboelectric charge, resistivity, softening point, blocking temperature, melt viscosity, and dispersion. Each of these parameters must be considered for each component in determining what components to combine and how to combine the components to achieve a balanced toner which produces an image having those properties determined to be most important for a specific toner. This choice of components is further influenced by economic and environmental concerns.
The bulk polymeric material of the toner generally functions as the binder for the colorants included in the toner formulation, but also affects many of the other toner functions, such as charging, electrical resistivity, and mechanical integrity, to name a few. Therefore, often times a combination of resins is used to achieve the desired performance. Polymers generally used in toner may be linear, branched or cross linked, and are chosen for their various properties and the manner in which these properties are likely to affect toner performance. For example, certain binder polymer properties affect the thermal performance of the toner. These properties include such binder parameters as glass transition temperature, melt viscosity, blocking temperature, and thermal integrity. In the same manner, the mechanical properties of the binder polymer, including such parameters as impact strength, adhesive/cohesive strength, and surface energy will also affect toner performance. Electrical traits such as triboelectric charge function, resistivity, and dielectric constant, and other miscellaneous features, such as moisture resistivity, % volatility, molecular weight, colorlessness, and pigment compatibility, all have an affect on the ultimate performance level of the toner in which the binder is used.
Among the most popular resins from which the toner resin may be selected are: acrylic resins, epoxy resins, polyamide resins, polyester resins, polyethylene resins, polystyrene resins, styrene-acrylic copolymer resins, and styrene-butadiene resins. As with all toner components, choice of resin is generally determined by the machine parameters and toner performance qualities sought.
Dispersed in the binder resin are the colorants used in the toner formulation. In monocomponent toners, magnetic oxide pigments are used for the purpose of enhancing the magnetic attraction between the toner and the developer roll assembly. Carbon black has historically been the most popular colorant used in black toners, as it strongly influences the triboelectric charging capability of the toner. However, more recent toners employ charge control agents to achieve and control this toner feature, thus allowing the use of more easily dispersed black colorants. The black colorant may also affect the flow characteristics of the toner and, therefore, is sometimes added in incremental amounts to the toner surface.
The charge control agents are also critical in full color printing. The equipment of today allows the reproduction of beautiful, photographic-quality full color images. The printer/copier machines generally employ one or more cartridges which dispense color toner, as well as black toner. The basic color toners used are magenta, cyan and yellow, though any number of other color toners are available. Generally, however, variations in color and tone or shade are produced by the combined printed affect of a basic color set of toners.
Most toner formulations also include any one or more of a number of materials known commonly in the industry as additives. These are generally fine particles which are physically blended with the toner. They may be attached to the toner by electrical means, mechanical means, or by mere physical mixing. These additives may be added to influence flow control, charge control, cleaning, fixing, offset prevention, transfer, conductivity control, humidity sensitivity control, and carrier life stability. Common additive materials include silica, metal oxides, metal stearates, fluoropolymer powders, fine polymer powders, rare earth oxides, waxes, conductive particulates, magnetite, carbon, and titanates. Choice of additives is critical, however, given that many of the additives affect more than a single toner property.
Clearly, given the vast number of components available in the industry for use in toner compositions, and given the propensity for many of the components to enhance some properties and at the same time to deleteriously affect others, choice of components is not a routine matter.
In addition to the content of the toner, toner preparation must address the problem of generating content-uniform particles exhibiting small particle size, in the range of about 2 μm to about 15 μm, preferably from about 5 μm to about 8 μm, and exhibiting a narrow particle size distribution. Shape of the particle can be equally important. The more uniform the shape and the smaller the particle size, the better the printed image. Several patents that disclose a means of controlling the shape and particle size of the toner particles include U.S. Pat. Nos. 4,833,060, 6,156,473, 6,294,595, and 6,380,297.
U.S. Pat. No. 4,833,060 discloses a process for producing powders of a controlled size and shape by dissolving a polymer in a solvent which is immiscible with water, forming a suspension of polymer/solvent droplets in the water, which contains a promoter and silica particles, and then removing the solvent from the polymer/solvent droplets, and drying and collecting the remaining polymer particles. These particles may then be used in toner. U.S. Pat. No. 5,049,469 also discloses a method of controlling particle size. In this patent, a latex copolymer is used as a stabilizer to control the size of the core in the shell-and-core toner particles disclosed. U.S. Pat. No. 6,294,595 teaches a process for preparing irregular-shaped toner particles by dispersing a polymer/organic solvent dispersion in an aqueous phase containing colloidal stabilizer particles with a positive charge and colloidal stabilizer particles with a negative charge. The use of only one or the other generates particles of a spherical nature, which this patent seeks to avoid. U.S. Pat. No. 6,380,297 achieves irregularly shaped particles by first creating spherical particles as taught in the processes just mentioned, and then surface-treating the particles with a surface-active agent to modify the particle shape.
Even if particle size and shape are controlled, uniformity and homogeneity of content remain a critical aspect of toner preparation. It is known, as was set forth earlier, to produce toner compositions which include pigment colorants. Such compositions may use carbon black. Other color toners may use color pigments commercially available from a number of sources. It is critical to the quality of the printed image, however, that the pigment or colorant used be homogeneously dispersed within the toner particles. This can be difficult to achieve given the propensity of pigment particulates to agglomerate, causing void areas in the toner particles which result in uneven color in the printed image. One means of dispersing a pigment in a toner composition is to use the pigment in the wet cake form. U.S. Pat. Nos. 5,667,929 and 5,591,552 disclose such a process for toner preparation. In these disclosures, pigment in the wet-cake form was added to a mixture of linear polyester and toluene to form a pre-dispersion. The water was flushed, or displaced, by a resin/toluene solution, and then the toluene removed to generate a crushed powder of resin and pigment. While this method does increase pigment dispersion to some degree, printed images using the toner nonetheless exhibit very average print quality.
Therefore, an aspect of concern, and the one of most importance to this invention, is that of pigment dispersion. In an optimum toner, each toner particle will be consistent with respect to performance, and will exhibit a uniform distribution of colorant, charge control agent, additives, etc. The degree to which this uniform dispersion is achieved affects the resulting triboelectric charge, color, yield, and finally the printed image.
One means to achieve homogeneous or uniform pigment dispersion is set forth in U.S. Ser. No. 10/878,860, filed Jun. 28, 2004, to our common assignee, the disclosure of which is incorporated herein by reference. In that disclosure, the pigment colorant is added to the toner composition in the form of an aqueous liquid pigment concentrate dispersion.
The current inventors have determined an alternative means by which to achieve uniform pigment dispersion within toner particles exhibiting volume average particle diameter of up to 15 μm. This is accomplished using an aqueous pigment pre-dispersion in combination with the processing steps set forth herein, to produce substantially spherical toner particles which have a uniform dispersion of particle components and are of small particle size and small particle size distribution, suitable for generating a printed image with enhanced brightness of colors, visual density and vividness of color. Each of the foregoing visible attributes is a direct result of the quality of the pigment dispersion incorporated into the toner.